In the description of the background which follows, reference is made to certain compositions, structures and methods, however, such references should not necessarily be construed as an admission that these compositions, structures and methods qualify as prior art under the applicable statutory provisions.
Ferritic materials of FeCrAl-type have good high temperature oxidation resistance properties but relatively low strength. It is known that high temperature strength and creep strength can be improved by preventing grain boundary slip through a combination of reduction of the grain boundary area and by adding material that prevents grain boundary slip and dislocation movements in the alloy.
Grain boundary slip is counteracted by a reduction in grain boundary area. One way of reducing grain boundary area is, of course, by increasing the grain size. Grain boundary slip can also be reduced by the introduction of stable particles, which counteract mobility of the grain boundaries. Such particles, which can be used in combination with reduced grain boundary area, have a size generally on the order of 50-1000 nm.
The high temperature strength of the alloy can also be improved by introducing a distribution of particles preventing dislocation movements. Particles used to this end should preferably have an average size of 10 nm or less, and be evenly distributed with an average distance of less than 200 nm. These particles must be extremely stable in relation to the metal matrix, in order not to be dissolved or coarsen with time. Suitable particle forming materials that counteract grain boundary slip and dislocation movements include stable nitrides of titanium, hafnium, zirconium and vanadium.
Consequently, it is known to nitride Fe and Ni based alloys containing stable nitride formers, such as Ti, and thereby create a dispersion of stable nitrides. Attempts have been made to nitride titanium containing FeCrAl-alloys in order to improve the high temperature and creep strength of these alloys. However, it has been established that the presence of Al, which is a fairly strong nitride former, results in a lowered solubility of nitrogen, which makes it difficult to transport nitrogen in the material. As a result, there is an inadequate amount of fine precipitation of titanium nitride. Furthermore, aluminum is bound in the form of aluminum nitride, which is harmful to the oxidation properties of the alloy. This aluminum nitride can be dissolved only at high temperatures thereby freeing up nitrogen for the formation of titanium nitride. However, titanium nitride formed in this manner becomes too coarse to effectively counteract dislocation movements. The presence of aluminum can further lead to precipitations of aluminum titanium nitride, which again is too coarse for the intended purposes.
In EP-A-225 047 a method to create a nitride dispersion by mechanically grinding powder containing a nitride former (preferably Ti) together with a nitrogen donor (preferably CrN and/or Cr2N) (so called MA-technique, where "MA" stands for Mechanical Alloying; see e.g., "Metals Handbook," 6th edition, volume 7, pp. 722-26). The grinding is carried out in a nitrogenous atmosphere. After grinding, the powder is heat treated in hydrogen gas to form titanium nitride and the nitrogen surplus is gassed off. The powder can then be consolidated by HIP'ping or extrusion. However, such alloys that do not contain aluminum have inferior oxidation properties at high temperatures when compared with FeCrAl-alloys.
In EP-A-256 555 an ODS-alloy (ODS: "Oxide Dispersion Strengthened") of FeCrAl-type is described. This alloy contains precipitations of a finely dispersed phase with a melting point of at least 1510.degree. C. The alloy consists of 20-30% Cr; 5-8% Al; 0.2-10 volume-% refractory oxides, carbides, nitrides and borides; &lt;5% Ti; &lt;2% Zr, Hf, Ta or V; &lt;6% Mo or W; &lt;0.5% Si and Nb; &lt;0.05% Ca, Y or rare earth metals; and &lt;0.2% B. The alloy is made by a grinding method (MA-technique). It is said to be very resistant to oxidation and corrosion up to 1300.degree. C. and to have good high temperature mechanical properties. However, the grinding process used to produce these alloys is very costly.
U.S. Pat. No. 3,992,161 describes FeCrAl-alloys with improved high temperature properties, whereby particles are ground into FeCrAl. The particles can include oxides, carbides, nitrides, borides or combinations thereof. Once again, the costly grinding process is utilized.
In the article of E. G. Wilson: "Development of powder routes for TiN dispersion strengthened stainless steels", Proceedings from the Conference on HNS 88 (High Nitrogen Steel 88), Lille, France, May 18-20, 1988, published by The Institute of Metals, England, an alternative method of achieving dispersion hardening is described, namely by precipitation of nitrides with high stability, for instance TiN. This method includes nitriding an alloy containing any element that forms stable nitrides. This nitriding is done in a fluidised bed and consolidation of the powder is accomplished by extrusion. The powder alloy is heated in a nitrogen-hydrogen gas mixture at 1150.degree. C. during formation of a dispersion of TiN-particles having a size of 50-200 nm. Surplus nitrogen is gassed off at the same temperature. In order to achieve the desired effect, the formed TiN-particles should be on the order of 20-30 nm in size. A prerequisite for formation of these fine TiN-particles is a high nitrogen activity, which can be achieved by a short diffusion distance and a high nitrogen gas pressure. The author suggests introduction of chromium nitride as a nitrogen donor. A high dissociation pressure is achieved by heating the chromium nitride to 1150.degree. C. However, these alloys contain no aluminum and therefore lack the appropriate corrosion properties. Furthermore the nitriding method is based on diffusion and is therefore inappropriate for thick walled sections since the ability of nitrogen to adequately penetrate deeply into the section is limited.
EP-A-161 756 relates to nitriding of a Ti-alloyed powder material in an ammonia/hydrogen gas mixture by formation of chromium nitrides in the form of a surface layer on the powder grains. The chromium nitrides can be dissolved at a higher temperature in an inert atmosphere, whereby nitrogen is set free, which then couples with titanium to form titanium nitride precipitations in the grains. Again there is no aluminum present which adversely affects corrosion properties.
EP-A-165 732 describes a method for making of titanium nitride dispersion hardened products. The nitriding is carried out on a porous powder body. Chromium and titanium containing iron or nickel base powder, which has gone through a soft sintering in hydrogen gas, is nitrided in a mixture of ammonia and hydrogen gas, so that chromium nitrides are formed on the free surfaces. Subsequently, a heat treatment in pure hydrogen gas at a higher temperature is carried out, whereby the chromium nitrides become disassociated, thereby freeing up nitrogen. Consequently, particles of titanium nitride are formed. The body is consolidated afterwards through extrusion, rolling or other methods. The disclosed alloy does not contain aluminum.
EP-A-363 047 describes the admixture of a nitrogen donor in the form of a less stable nitride, usually chromium nitride, in a powder containing a nitride former. Nitrogen is liberated from the donor by heating and can then react with the nitride former in the powder, so that fine nitrides are precipitated. Treatment of titanium containing FeCrAl-powder with this method results in the precipitation of aluminum nitride, which is difficult to dissolve, rather than a primarily titanium nitride containing powder. The aluminum nitride can be dissolved at high temperature and form titanium nitride, but as mentioned above, this leads to the formation of titanium nitride and to the precipitation of aluminum titanium nitride.
GB-A-2 156 863 relates to a titanium nitride dispersion hardened steel. This method describes a process to make a titanium nitride dispersion hardened powder-metallurgy alloy of stainless steel, or nickel-based alloy, containing titanium. The process includes heating of the alloy in ammonia to about 700.degree. C., whereby the ammonia gas disassociates and a layer of chromium nitride is formed on the surface of the powder grains. The chromium nitride is dissociated in a subsequent step in a mixture of nitrogen gas and hydrogen gas after rapid heating to a temperature of 1000-1150.degree. C., whereby titanium nitride is formed. This method results in great amounts of atomic nitrogen corresponding to a very high nitrogen activity level. The heat treatment continues after the formation of titanium nitrides as the composition of the gas simultaneously is changed to pure hydrogen gas for removal of surplus nitrogen. Since this method involves the treatment of FeCrAl-powder in a nitrogen-rich environment as described above, aluminum nitride is precipitated. As previously noted, this aluminum nitride compound is difficult to dissolve. While the compound can be dissolved at high temperature to form titanium nitride, the disadvantageous coarsening of the resulting titanium nitride, as well as the disadvantageous precipitation of aluminum titanium nitride results.
Further nitriding methods are described in EP-A-258 969, GB-A-2 048 955, U.S. Pat. No. 3,847,682, U.S. Pat. No. 5,073,409 and U.S. Pat. No. 5,114,470, and in ASM Handbook, volume 4, 1991 edition, pages 387-436.
When applying nitriding methods according to above on aluminum oxide forming high temperature alloys, nitrogen will preferably be bound as aluminum nitride. This leads to two drawbacks. First, that the ability of the alloy to form a protective aluminum oxide is limited. Second, the formed nitrides become too big and are not stable enough.
Therefore, it would be advantageous to be able to form an alloy with good oxidation resistance, as well as good high temperature strength and creep resistance, in a cost effective manner.